1. Field of the Invention
This invention relates to terrain aided navigation for airborne vehicles, and more particularly to the use of multi-channel monopulse radar imaging having off-axis capability for terrain aided navigation.
2. Description of the Related Art
Long-range navigation such as for satellites, manned or unmanned aircraft or missiles is commonly accomplished by providing an initial position, velocity and acceleration (“PVA”) and attitude, using inertial sensors on-board the air vehicle to provide inertial measurements and integrating those measurements to update a navigation PVA and attitude over time. The inertial measurements may be provided by acceleration and angular rate sensors such as in an Inertial Measurement Unit or IMU. The navigation PVA and attitude include an error component that is bounded by an uncertainty region. The uncertainty region is dependent on several factors including the accuracy of the initial PVA and attitude, the quality of the inertial sensors (which tend to drift over time), and the accuracy of the clock. Without correction, the uncertainty region of the navigation PVA will continue to grow over time.
A Kalman Filter is typically used to reduce the noise and some of the bias errors in the integrator's estimate of PVA. The noise reduction and bias corrections a Kalman Filter provides are limited to the errors that are observable with the vehicle's IMU and a priori knowledge of the platform's dynamics in the form of a “motion model”. Kalman filtering reduces but does not eliminate the growth in the uncertainty region.
Terrain aided navigation has existed for serveral decades. Periodically measurements of the terrain are taken and correlated to a terrain database to correct the navigation position estimate and reset the uncertainty region.
U.S. Pat. No. 3,328,795, Terrain Contour Matching (TERCOM), first developed in the 1950's, uses radar altimeter measurements to compute a history of terrain heights and correlates that history with terrain heights from a database map. TERCOM requires labor-intensive preparation to tailor a set of terrain maps for each mission. TERCOM only works when there is terrain contour below the own-airborne vehicle. The missile is constrained to straight flight at a certain speed while taking the measurements. A further constraint stems from the assumption that the radar altimeter only provides the range to the nearest terrain directly below the vehicle (within the radar altimeter's antenna beamwidth).
U.S. Pat. No. 7,522,090 suggests a modification to TERCOM that allows for the use of a terrain height database that can be formatted independently of the flight path. A reference basket is calculated onboard from the horizontal position uncertainty, and sample points comprising the horizontal position and altitude sample (i.e. the difference between the vertical inertial altitude and the clearance altitude at the horizontal position) are correlated over the reference basket. The sample history continues to grow until a satisfactory correlation can be achieved. The reference basket is a set of cells in the terrain database and remains constant over the history.
Honeywell developed an interferometric radar sensor and associated algorithm called Precision Terrain Aided Navigation (PTAN), patented under U.S. Pat. No. 6,512,976. PTAN provides more precise measurements, and in particular provides not only the range to the nearest terrain, but also bearing angles. Locating a terrain feature with an accuracy that is finer than the resolution of the terrain database allows for a more accurate position fix. PTAN requires three antennas with a significant spatial separation to get the full angular information, which limits its use to vehicles that are large enough to accommodate this spacing, and requires expensive calibration of the boresight angles to a high accuracy.
U.S. Pat. No. 4,910,674 Navigation of Aircraft by Correlation proposes a method of navigation in which the radar altimeter (the millimeter wave sensor operating at about 94 GHz) is scanned back and forth in a substantially vertical down direction across a prescribed flight path to measure altitude (see FIG. 2). The sequence of altitude measurements is fed to the correlator.
U.S. Pat. No. 6,233,522 B1 proposes a radar that gathers terrain data that is compared to a stored terrain data base using a test statistic. The test statistic can be used to validate the terrain data base information and/or the aircraft position data. There is no way to definitely confirm an aircraft's reported position due to the statistical nature of the radar signals. However, by accumulating evidence from a variety of positional systems, it is possible to reject the aircraft's reported position as an unlikely event given active sensor data and DTED data. The method is based on statistical concept of hypothesis testing. A test statistic is defined that is used to determine if there is sufficient evidence to reject the conjecture that the information in the DTED along with the outputs from the aircraft's NAV, and FMS are consistent with the active sensor readings. This method is applicable to real beam radars, monopulse radars and laser radars (Col 3, lines 35-47).